1. Field of the Invention
The present invention relates to an image forming apparatus and a conveyance malfunction decision method in an image forming apparatus, and more particularly to an image forming apparatus for forming images through electrophotographic process such as a copying machine and printer, and to a conveyance malfunction decision method in the image forming apparatus.
2. Description of Related Art
Conventionally, an image forming apparatus has a sensor mounted on a conveyance path, make it detect the paper length, and if the size of paper actually placed in the paper feed section differs from that of paper designated to be printed, it decides that a paper jam occurs and reports it. For example, if a user fails to adjust a sliding piece in a paper cassette that automatically detects the paper size, it might occur that the automatically detected paper size differs from the real paper size. In this case, a decision is made that a conveyance malfunction (also referred to as a paper jam) occurs if the length detected by the sensor on the conveyance path differs from the size automatically detected by the cassette (see Japanese Patent Laid-Open No. 09-40217/1998, for example).
Besides, an image forming apparatus is proposed which circumvents a paper jam by employing a control method that detects the length of only the first sheet of printing while feeding and conveying it, and then feeds the second and subsequent sheets according to the paper length detected (see Japanese Patent Laid-Open No. 2002-154682, for example).
Recently, however, to further increase the printing efficiency of the image forming apparatus, improvement has been required in the printing speed (the number of sheets printed in a unit period of time: referred to as “throughput” from now on). In the foregoing control method that detects the length of only the first sheet while feeding and conveying it, and then feeds the second and subsequent sheets, the second sheet cannot be fed until completing the detection of the length of the first sheet. Thus, the method cannot achieve the improvement in the throughput. Accordingly, to improve the throughput, it is necessary to start the feed and conveyance of the second sheet before completing the detection of the length of the first sheet while feeding and conveying it. In this case, a problem can arise of causing a paper jam because of the difference in the size between the paper actually placed and the paper designated to be printed.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to reduce the conveyance malfunction as much as possible when a real sheet differs in size from the sheet set to be printed, while increasing throughput by starting the feed and conveyance of the second sheet before the detection of the paper length of the first sheet has been completed.